The position of a mobile device can be determined in several ways. One option is using a radio-map based positioning.
Radio-map based positioning techniques utilize information on the coverage area of a transmission node for determining the position of a mobile device. A transmission node can be any node that transmits signals on the radio interface and that can be identified by the transmitted signals. It could be for instance a communication node of a cellular network or of a wireless local access network. The coverage area of a transmission node is the area in which a mobile device may hear signals from the transmission node. Alternatively, the coverage area may be limited based on various criteria. For instance, the coverage area may represent an area, in which the communication node can be heard at signal strength above a predetermined threshold signal strength.
A position of a mobile device can be determined to correspond for instance to the coverage area of a transmission node of which the mobile device is currently receiving signals. Alternatively, it could be determined to correspond to the center of the coverage area, with a maximum error indicated to be the maximum distance from the center to the boundary of the area, etc.
In case signals from multiple transmission nodes are detected by a mobile device, their coverage areas may be used in combination to find the most likely location of the mobile device. The location can be determined, for instance, to lie in the intersection of the coverage areas, at a weighted average of the coverage areas, or similar. The coverage areas may be given different weights depending on, for instance, the size of the coverage area or the observed signal strength.
A radio-map comprises models of the coverage area of several transmission nodes. A model of the coverage area of a transmission node may be determined in various ways.
If the location of a transmission node is known, a coverage area model can be formed using radio propagation models. If available, some additional information can be used, such as antenna azimuth and beam width. Moreover, some information about the environment can be utilized such as urban/sub-urban/rural. Furthermore, the landscape and the frequency affect the propagation and can be considered when using a radio propagation model.
Alternatively, a coverage area model can be formed using fingerprints. Fingerprints are samples that are collected by mobile devices with some independent positioning capability, such as an assisted global navigation satellite system (AGNSS) based positioning capability. A fingerprint may contain the location of the fingerprint and a list of the identities of transmission nodes from which signals are heard at the indicated location. The identity of a transmission node may be for instance a wireless local area network (WLAN) access point (AP) media access control (MAC) address or the cell-identity (ID) of a cell of a cellular communication network. In addition, a fingerprint may contain some other information, such as observed signal strength values.
Collected fingerprints can be used in various ways to construct a coverage area model for a transmission node.
A coverage area model may simply represent the maximum area in which signals of a transmission node have been detected according to the fingerprints. In this case the position estimate could lie in the intersection of coverage areas.
Another option is to model the coverage areas statistically to obtain a probability distribution of the coverage areas. In this case the intention is not to model the actual coverage area of the communication node, but in fact the distribution of the users within the true communication node coverage area. It may be assumed to this end that the location of mobile devices in the coverage area of a transmission node is normally distributed, with a mean corresponding to the mean of the location of the collected fingerprints and with a variance corresponding to the variance of the collected fingerprints. In such a case, the position estimate could be the maximum likelihood estimate of the joint probability distribution of the coverage areas of all transmission nodes from which a mobile device that is to be positioned receives signals.